Adding a coax relay is not hard. I don't get why its such a big deal
to power preamps thru the coax. Serious VHFers do not do this but
run separate power wiring to the preamp.
BTW you can leave preamps powered all the time (even when
transmitting); GasFets are more immune to burn out when biased on. I
am talking about a small amount of RF leakage thru the coax
relay. This applies only for simple preamps and not the RF switched
versions. Most of those sense RF and switch to bypass in
transmit. Transmitting directly into a simple preamp will destroy it.
If you are running a transceiver with only a single antenna connector
for receive and transmit, you will need two coax relays: one on the
antenna side of the preamp and one on the radio side. In transmit
the relays bypass the preamp with a short coax jumper. The coax
relays will need to be controlled by the radio's PTT control
line. Most high-level muli-mode radios have this available on an
accessory jack. I do not know if this is normally available on
FM-only VHF/UHF radios and you will have to look in the manual for
your radio to determine this.
If you must power your preamp thru the coax then DEMI makes a bias
inserter. If you do this then you will need to pick off 12v from the
inserter to power the preamp and relays. Configure the relays to
bypass when unpowered. Again, this does not apply to RF switched
preamps. As far as I know most radios are unable to supply
sufficient current to power a relay. I think powering preamps with
your radio up th ecoax mostly applies to the RF-switched versions.
As a side note: I have added a webpage to show how to provide a
direct rx antenna connection on the FT-847:
http://www.kl7uw.com/FT-847RXANT.htm
It refers to article done by DL4MEA.
73 Ed - KL7UW
At 06:53 PM 4/19/2008, Michael Pfeuffer wrote:
>DEMI makes a couple of nice pre-amps, but they don't have switching and
>require external power. Has anyone tried to add a T/R switch & coax bias
>to mast mount them?
>>--Mike WQ5C
>>>At 03:23 PM 4/18/2008, Michael Tondee wrote:
> >I just got the Icom AG-35 to use with my IC-820H. It's a bit cheaper than
> >the ARR and SSB electronics units and handles 100 watts. I know that it
> >doesn't have as good a noise figure as the ARR and SSB electronic units but
> >I couldn't afford them and I got the Icom at a discount from a ham store I
> >used to work for. I used to have an ARR 160 rated watt mast mount and sold
> >it when I sold my TS-2000X. I'm just now about to install the AG-35 so I
> >hope it helps. One things for sure, it's tough sledding without a preamp on
> >70cm.
> >73,
> >Michael, W4HIJ
> >----- Original Message -----
> >From: "Mike Ryan" <mryan301 at comcast.net>
> >To: <amsat-bb at amsat.org>
> >Sent: Thursday, April 17, 2008 11:13 PM
> >Subject: [amsat-bb] Any feedback on Mirage KP1-440 Preamp?
> >
> >
> > > Hello all,
> > >
> > > Zapped my ARR RX only preamp for the second time out of stupidity. Time
> >for a switched model. Cant afford the $200+ amps from SSB and such. The
> >Mirage seems attractive. Can anyone share their experiences with this model
> >or suggest a differnt affordable preamp? I looked at the ARR switched models
> >already. The high powerd mast mount is too much $$ and the low power one
> >only handles 25 watts. I could easily smoke that one out of stupidity as
> >well (my rig can do 50 watts).
> > >
> > > Mike WB1AAT
> >
> >_______________________________________________
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